1. Field of the Invention
This invention relates to an optical system for a multidetector array spectrograph to be used as a component of a spectrophotometer capable of measuring the absorbance of light by a sample as a function of the wavelength of light passing through the sample. This invention further relates to an optical system for a multidetector array spectrograph to be used as a component of a radiometer or emission spectrophotometer capable of measuring light energy emitted by a sample as a function of the wavelength of light. This invention further relates to an optical system for a multidetector array spectrograph to be used as a component of a reflectance spectrophotometer capable of measuring the reflectance of light from a sample as a function of light wavelength. This invention further relates to an optical system for a multidetector array spectrograph to be used as a component of an instrument capable of measuring the fluorescence or phosphorescence of a sample. This invention further relates to an optical system for a multidetector array spectrograph to be used as a component of an instrument capable of measuring the emission spectra or radiated energy of a sample as a function of wavelength.
2. Description of the Prior Art
For some years, it has been known that meaningful laboratory analysis can be performed using instruments which measure the light absorbed by a sample, or reflected from a sample, or emitted from a sample as a function of wavelength. Spectrophotometers which measure the absorption characteristics of materials were produced by Bausch & Lomb in 1953. The absorption may be indicative of the presence of an impurity in a liquid under test, of solute in a solvent, of the color of the liquid, of the presence of solid matter suspended in the liquid, or the like. Numerous instruments for such applications are known. The art has well documented the wavelengths of light which are absorbed by various materials so that the absorption of light of a specific wavelength is indicative of the presence of a particular material in the sample under test. If the amount of incident light and transmitted light are compared, an indication of the amount of the absorptive material may be derived.
For these reasons, it is often desirable to make measurements of the amount of light that a sample absorbs as a function of the wavelength of light. It has also proven desirable to measure light absorption by a sample for selected ranges of wavelengths of light. Prior approaches to measuring light absorbed by a sample as a function of wavelength have typically utilized one of two very well known techniques.
One prior art technique is to generate white light and direct the generated white light into a monochromator. The monochromator receives white light emitted by the source and produces a monochromatic light of a selected wavelength by allowing only the small band of selected wavelengths to emerge from the exit slit of the monochromator. The light emerging from the monochromator travels through a sample under investigation. Typically, a portion of the light entering the sample would be absorbed by the sample itself. The remaining monochromatic light passing through the sample is measured by a single detector placed on the other side of the sample cell. In order to measure characteristics of the sample to absorb light over a range of wavelengths, the above experiment would be repeated for each wavelength in the selected range of wavelengths by adjusting the grating in the monochromator such that a next wavelength in the selected range would be emitted. Measurements would be repeated for each wavelength within the selected range. Such a procedure would successfully permit an examination of sample absorbency of light over a range of wavelengths. One such spectrophotometer has been marketed by the Milton Roy Company under the name "SPECTRONIC.TM. 2000". The primary disadvantage to devices which use such an approach is that when measurements of sample absorbency for a relatively large wavelength range is desired, the repeated adjustments to the monochromator would result in a relatively lengthy time to acquire data over the desired wavelength range.
The second technique to measure sample absorption over wavelength ranges is to generate and transmit white light directly through the sample under investigation. Light passing through the sample is directed to a spectrograph where an array of photodectectors would simultaneously. In order to get high resolution of sample absorption readings over the large wavelength range typically present in such a method, various approaches have been taken.
One approach for measuring over a wide range of wavelengths is to use a detector array with a very large number of detecting elements in conjunction with a fixed grating and entrance slit. Unfortunately, such detector arrays tend to be very expensive, and therefore undesirable for many uses. Also, as detector arrays are planar, focus of the spectrum over the long array length is inherently poor.
Another approach includes the use of an array where the range of wavelengths studied could be changed by rotating the grating to a new location. This approach has proven undesirable as the mechanical system which positions the grating tends to adversely affect the accuracy of the device. Thus, to get repeatable results, the mechanical system which utilizes the reduced size detector array must be able to locate and position the grating with a high degree of precision. One such spectrophotometer has been marketed by Perkin-Elmer.
Yet another approach was to direct light from the entrance slit onto two or more gratings. Light from the gratings would be directed onto a corresponding sensor array. Such a system requires the proper alignment of numerous mechanical and optical components and proved too expensive for many spectrophotometric applications. One such spectrophotometer has been marketed by Hewlett Packard as their model 8450A spectrophotometer.
All of the above described techniques involve the use of a spectrograph where a single slit or aperture of various shapes is provided for light to pass through in combination with various dispersive and focusing systems for collecting light from the slit, dispersing the light by wavelength and refocusing the light at detectors. The limitation of the single slit designs described above is that resolution is limited by the number of detectors. It has proven difficult and expensive to obtain the electronics for a large number of detectors and to form an accurate high resolution image over the larger range of positions required for a large number of detectors.
Furthermore, if the detectors are small and close together, it may become optically difficult to disperse the wavelengths accurately without mixing in light from incorrect wavelengths. This is one source of stray light which is a type of error in dispersive instrumentation. Since gratings disperse light by means of interference effects, gratings image some light of wavelength lambda, one half lambda, one quarter lambda, etc, in the same place. When an instrument is designed such that the longest wavelength to be analyzed is more than twice the shortest wavelength to be analyzed and a grating is used as the dispersive element, then order filtering must be used to suppress the light from the half wavelength values which would normally reach the detector. For example, in a system designed to detect wavelengths from 400 to 900 nm, the detectors between 800 and 900 nm would see some light from wavelengths between 400 and 450 nm. This problem is normally addressed by placing additional filters in the system which transmit light at the desired wavelength and absorb light at the half wavelength. Such filters may be inserted into the light path by moving mechanical means or may be inserted into the dispersive and focusing system to intercept light rays reaching the longer wavelength detectors only. These filters are generally called order sorting filters. If the additional filters are used improperly or scatter light within the dispersive and focusing system, then they become another source of stray light. If the wavelength range of the instrument is broad enough, it becomes difficult to fabricate optical elements which perform well over the total range of wavelengths, thus forcing the sacrificing of optimal performance in some areas to obtain acceptable performance over the complete range.